If this property holds in a system, the total von Neumann entropy may be less than the sum of its components. This property was explained away in a thought experiment that promoted a hidden variable theory of physics by showing that information must travel faster than light 
If this property holds in a system, the total von Neumann entropy may be less than the sum of its components. This property was explained away in a thought experiment that promoted a hidden variable theory of physics by showing that information must travel faster than light or violate locality, the "spooky action at a distance" in the words of one of that experiment's formulators. Experiments by Aspect and others have confirmed this property's existence by violating Bell's inequalities, which should be followed if a hidden variable that Einstein, Podolsky and Rosen predicted exists. For 10 points, name this relation between particles where the description of one cannot be known without also knowing the other.
If this property holds in a system, the total von Neumann entropy may be less than the sum of its components.
{quantum entanglement} [prompt on “{EPR Paradox}” or “{EPR effect}” before {being} mentioned]